ABSTRACT Most organisms, including humans, show daily rhythms of about 24 hoursin physiology, hormonal function, and behavior. In mammals, these rhythmsare controlled by an endogenous circadian pacemaker localized in the suprachiasmaticnucleus (SCN) of the hypothalamus that determines thetemporal organization of several behaviors and physiological processes.Circadian control of daily rhythms differs in diurnal and nocturnal speciesbut many of the mechanisms that may explain these differences remain stillunknown. The aim of this review is to summarize our current knowledgeof the circadian clocks and the differences between diurnal and nocturnalspecies.